Many waters contain alkaline earth metal cations, such as barium, strontium, calcium, magnesium and anions, such as sulfate, bicarbonate, carbonate, phosphate and fluoride. When combinations of these anions and cations are present in concentrations which exceed the solubility product of the various species which may be formed, precipitates form until the respective solubility products are no longer exceeded. For example, when the concentrations of the barium and sulfate ions exceed the solubility product of barium sulfate, a solid phase of barium sulfate will form as a precipitate. Solubility products are exceeded for various reasons, such as evaporation of the water phase, change the pH, pressure or temperature and the introduction of additional ions which can form insoluble compounds with the ions already present in the solution.
As these reaction products precipitate on the surfaces of the water-carrying or water-containing system, they form adherent deposits or scale. Scale may prevent effective heat transfer, interfere with fluid flow, facilitate corrosive processes, or harbor bacteria. Scale is an expense problem in any industrial water system, in production systems for oil and gas, in pulp and paper mill systems, and in other systems, causing delays and shutdowns for cleaning and removal.
In our U.S. Pat. Nos. 4,980,077, 4,990,718, 5,049,297 and 5,084,105, there is disclosed a method for removing barium sulfate and other sulfate scales by a solvent comprising a combination of a chelating agent comprising a catalyst or synergist comprising polyaminopolycarboxylic acid such as EDTA or DTPA together with anions of (1) a monocarboxylic acid such as acetic acid, hydroxyacetic acid, mercaptoacetic acid or salicylic acid; (2) oxalates; (3) thiosulfates or (4) nitriloacetic acid. The scale is removed under alkaline conditions, preferably at pH values of about 8.0 to about 14.0, with best results being achieved at about pH 12. When the solvent becomes saturated with scale metal cations, the spent solvent is disposed of by re-injection into the subsurface formation or regenerated.
It is common practice when using scale dissolvers to pump a slug or volume of solvent into the well and leave it to stand static for a long period of time. This is not effective from an economic stand point because the well must be shut in for long periods of time, resulting in loss of production. Soaking is also not an efficient method from a reaction rate standpoint. Also, circulating scale solvent into the well bore is too costly, due to the large volume of solvent needed to fill the tubing and allow circulation.
The article by A. D. F. Brown, S. J. Merrett, and J. S. Putman, entitled "Coil-Tubing Milling/Underreaming of Barium Sulphate Scale and Scale Control in the Forties Field", published by the Society of Petroleum Engineers in 1991 discloses milling barium sulphate scale from the interior walls of downhole production tubing using coiled tubing with a downhole motor which rotates an underreamer and/or bit.
This invention provides an effective method for removing alkaline earth metal scales from the interior surface of downhole production tubing by circulating a scale-removing solvent into the production tubing while simultaneously milling the scale walls with a rotating working drill or mill head connected to a downhole motor.